What is meant by “altitude above sea level” and how is the zero reference determined. Is it a constant with respect to space and time?
Heights above sea level, like mountains or whatever, have traditionally been defined (and still are in most cases) in terms of a measurement of ‘mean sea level’ at one or more locations. That mean sea level, once determined at the location, is then carried around the country by means of surveyors’ spirit levelling apparatus (i.e. better versions of the levelling apparatus you must have seen surveyors use in the road or construction industry). So, for example, here in the U.K. we define heights above sea level in terms of ‘Ordnance Datum Newlyn’ (ODN) which is the mean level of the sea at Newlyn in Cornwall in S.W.England in the period May 1915 to April 1921 (which replaced an earlier Ordnance Datum Liverpool based on sea level in that port in 1844). That level has been carried around the country by surveyors’ methods (i.e. levelling, which is equivalent to imagining a set of ficticious thin canals with no flow permeating the country which allow the ODN level, the ‘zero’ reference level
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